(Not Applicable)
Radar systems use time delay measurements between a transmitted signal and its echo to calculate the range to a target. Ranges that change with time cause a Doppler offset in phase and frequency of the echo. Consequently, the closing velocity between target and radar, which is also known as radial or line-of-sight velocity, is conventionally measured by measuring the Doppler offset of the echo. In a pulse-Doppler radar, Doppler frequency is measured as a linear phase shift over a set of radar pulses during some Coherent Processing Interval (CPI). The average time delay over the set of pulses corresponds to the reported range of a target.
Radars that detect and measure target velocity are known as Moving-Target-Indicator (MTI) radars. MTI radars that are operated from aircraft are often described as Airborne-MTI (AMTI) radars. When AMTI radars are used to detect and measure ground-based moving-target vehicles, they are often described as Ground-MTI (GMTI) radars. Good introductions to MTI radar operation are given in M. I. Skolnik, Introduction to Radar Systems, second edition, ISBN 0-07-057909-1, McGraw-Hill, Inc., 1980 and F. E. Nathanson, Radar Design Principles, second edition, ISBN 0-07-046052-3, McGraw-Hill, Inc., 1990. MTI systems with exceptionally fine range resolution are known as High Range Resolution (HRR) systems. HRR systems should have range resolution finer than the dimensions of the target vehicles to facilitate vehicle discrimination and identification.
Radar systems that employ the same antenna for transmitting and receiving echoes are termed monostatic radars. When transmit and receive antennas are separated, it is termed bistatic operation. Although two spaced antennas on a single platform could be bistatic, typically antennas for a bistatic system are on separate vehicles where both vehicles move with respect to each other and the target.
In all cases, pulse-Doppler radars require a multitude of pulses for processing to extract closing velocity information. Conventionally, the Pulse-Repetition-Frequency (PRF) is held constant over a CPI. Furthermore, the pulse waveform parameters of center frequency and chirp rate are held constant. This suffices since typical GMTI radars have generally coarse range resolutions (typically several meters to several tens of meters), and relatively short CPIs (typically small fractions of a second). Constant waveform parameters simplify the signal processing to extract range and velocity information.
A typical output of a GMTI radar is a range-Doppler map that is a plot of target range from the radar as a function of target velocity (in the range direction) with respect to the radar. With finer range resolutions and longer CPIs (especially with higher radar platform velocities) of modern high-performance GMTI systems (especially HRR GMTI systems), it is difficult to optimally focus the range-Doppler map and maximize the probability of detecting targets and identify their range and radial velocity. Target vehicles migrate through range and Doppler/velocity cells during the course of data collection. This movement, which is related to the range-migration and motion compensation problems in Synthetic Aperture Radar image formation, shows up on the range-Doppler map as a smear, rather than a desirable point.
An airborne GMTI system emits pulses and collects data while the radar is in motion; i.e., flying along a flight path. Each pulse represents data taken not only at a different time, but also taken from a different spatial location due to the motion of the aircraft. Consequently, a target with some constant closing velocity towards one pulse""s spatial location will exhibit a different closing velocity towards a different pulse""s spatial location. As a result, even for a constant velocity target, Doppler frequencies are not constant during the data collection interval along the flight path, and exhibit a migration of values as data is collected. Doppler resolution and estimation (which corresponds to velocity resolution and estimation) is therewith impaired.
R. W. Wills et al., U.S. Pat. No. 6,307,501, Oct. 23, 2001, xe2x80x9cRadar systemsxe2x80x9d, mention motion compensation for GMTI radar, but only in the context of centering a clutter map, and not for correcting migration effects. Waveform parameters such as chirp rate and center frequency are not discussed.
Similarly, T. L. ap Rhys, U.S. Pat. Nos. 4,093,950 and ""951, Jun. 6, 1978, xe2x80x9cMotion-compensation arrangements for MTI radarsxe2x80x9d, also offers motion compensation during GMTI echo processing for multiple antenna arrays, employing xe2x80x9cphase and amplitude adjustmentsxe2x80x9d, as well as delays, to stabilize clutter in the direction of the antenna boresight. Migration effects are not addressed.
L. R. Flumerfelt et al., U.S. Pat. No. 5,163,176, Nov. 10, 1992 xe2x80x9cAll weather tactical strike system (AWTSS) and method of operationxe2x80x9d, offer motion compensation by received signal xe2x80x9cphase rotationxe2x80x9d. Constant phase rotations or shifts do not address problematic migration for the radar systems addressed by this disclosure.
D. C. Schleher, MTI and Pulsed Doppler Radar, ISBN 0-89006-320-6, Arthech House, Inc., 1991, indicates that the principal use of motion compensation is to force clutter responses to an effective Doppler frequency of zero. Migration is not addressed.
Bistatic GMTI systems are also addressed by M. W. Long, U.S. Pat. No. 4,459,592, Jul. 10, 1984 xe2x80x9cMethods of and circuits for suppressing Doppler radar clutterxe2x80x9d and U.S. Pat. No. 4,684,950, Aug. 4, 1987, xe2x80x9cMethods of and circuits for suppressing Doppler radar clutterxe2x80x9d. Long is principally concerned with clutter suppression and defers any detailed discussion of motion compensation, stating techniques xe2x80x9care a well-established part of the radar artxe2x80x9d.
O. J. Jacomini, U.S. Pat. No. 4,048,637, Sep. 13, 1977, xe2x80x9cRadar system for detecting slowly moving targetsxe2x80x9d, reveals a bistatic motion compensation scheme to control clutter spectrum width by forcing separate aircraft to fly opposite angular velocities. However, no mention of migration effects is made. Waveform parameters such as chirp rate and center frequency are not discussed.
It is an object of this invention to adjust some combination of transmitted waveform parameters and digital sampling parameters as a function of the instantaneous geometry of the data collection to focus the range-Doppler map of a GMTI radar.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, a method is provided of compensating for defocusing of range-Doppler map in a GMTI video signal caused by movement of radar during a coherent processing interval over which a set of radar pulses are processed. Waveform or sampling parameters of each pulse are varied to compensate for distortions caused by changes in viewing angles from the radar to the target.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.